Encyclopedia of Evolution.pdf - Online Reading Center

0 vestigial characteristics
algae. The chloroplasts of dinoflagellates not only have extra
membranes but even have nucleomorphs, which are vestigial
nuclei left over from the eukaryotic algal ancestors!
Examples at the organ level are almost innumerable.
At the organ level, in plants. Staminodes, mentioned previously,
are one example—or are many examples, since they
have evolved independently in several different plant families.
Also, grasses have flowers, but as these flowers are wind-pollinated,
they have no use for petals. Grass flowers do, however,
have tiny vestigial petals called lodicules.
At the organ level, in animals. Various animal species
have structures that are usually considered vestigial. Examples
include:
• The appendix in humans is a remnant of the cecum, a
pouch of the intestine that has an important function in
animals that have a high food intake of plant materials.
• Flightless birds have wing remnants. Ostriches have wings,
but they are small and not used for flight; kiwis have vestigial
wings so small that they have no important function.
• Some snakes, and some whales, have vestigial hip and leg
bones, even though they do not have legs (see whales,
evolution of). The ancestors of horses had several toes,
while modern horses have just one (the hoof); occasionally,
modern horses develop extra toes, because they still have
the vestigial, unused genes for these extra toes.
• Some toothless animals have teeth during their embryonic
stages (for example, baleen whale embryos develop teeth,
which are then reabsorbed before birth) or have the genes
for teeth but do not use them (for example, chickens).
• Male mammals have nipples. Male nipples are not themselves
so much vestigial as they are a side effect of the
development of nipples in female mammals (see developmental
evolution). The fact that nipple production is
not prevented during male development can be considered
a vestigial characteristic.
• Blind cave fishes may have no eyes but still have eye sockets.
Tissue transplants, and crosses between subspecies
of blind fish, result in small but still functional eyes. This
demonstrates that blindness evolved recently in these fish
populations.
Another example of a vestigial characteristic is patterns
of seasonal activity that make sense only when one considers
the evolutionary past. Deciduous trees open their buds
in the springtime. Some tree species (for example, elms and
maples) open their buds early, when there is still some danger
of frost; other tree species (for example, persimmons)
do not open their buds until nearly all danger of frost is
past. These trees deal with the danger of frost in two different
ways: The trees that open their buds early must tolerate
the frost, while the trees that open their buds late are
avoiding the frost. Tolerance requires the trees to produce
protective chemicals within their buds, but in return, they
are able to utilize the sunshine of early spring. Avoidance
requires the trees to lose the opportunity to utilize early
spring sunshine, but in return, they do not need to produce
protective chemicals. In general, trees that have evolved
in cold climates (such as northern North America) tolerate
frost, while trees that have evolved in warm climates
(such as southern North America) avoid frost. Trees that
are native to the middle latitudes of North America (for
example, in Oklahoma) have a mixture of tree species that
tolerate and avoid frost; both methods should work equally
well. It just so happens that the tree species native of Oklahoma
that tolerate frost are members of plant families that
evolved in the north, and trees species native to Oklahoma
that avoid frost are members of plant families that evolved
in the south, during the early Cenozoic era in the northern
continents. Some trees tolerate, some avoid, frost, not
because of current climatic conditions, but because they
have inherited the evolutionary adaptations of their ancestors.
This evolutionary pattern can be considered vestigial,
left over from the evolutionary past.
The primary assumption behind creationist attacks on
the concept of vestigial characteristics is that an organism
designed by a higher intelligence cannot have useless characteristics
(see intelligent design); biologist Michael Denton
went so far as to say that there can be no exceptions. Therefore
they have attempted to prove that characteristics which
evolutionary scientists have ever identified as vestigial are, in
fact, useful. The problems with their approach include:
• They assume that, if they demonstrate a function for the
vestigial characteristic, they have shown it to be useful.
Some creationists have argued that the human appendix
is part of the lymphatic system. While the appendix does
have a great deal of lymphatic tissue, this does not make it
an important part of the lymphatic system. The lymphatic
tissue of the appendix is important because the appendix
traps bacteria and is prone to infection; the lymphatic tissue
is therefore important because the appendix is vestigial,
not in spite of it. Creationists also argue that nipples on
male mammals are, in fact, useful, because under some circumstances
males have been known to lactate (for example,
from a surge of estrogens resulting from food intake
following near starvation). Yes, this is a function, but can
it really be considered useful? Some creationists have also
made the claim that nipples in male mammals are useful
because they contribute to erotic stimulation. While this
is true, erotic stimulation is not the major function of a
nipple, nor is such a complex structure necessary for erotic
stimulation. Staminodes in flowers can be colorful, like
petals, and help to attract pollinators, but this cannot be a
primary or necessary function. Of course, with enough special
pleading, a scenario of usefulness can be constructed
for almost anything.
• They assume that, if they make a credible argument for one
component of a category, they have proved the usefulness
of the entire category. Perhaps the major example of this is
their argument that the usefulness of some of the noncoding
DNA demonstrates that none of the noncoding DNA is
vestigial.
• In some cases, the potential usefulness of the vestigial characteristic
is for evolution itself. So-called junk DNA may
be very useful as a potential future source of genetic vari-